In the telecommunications field, optical fibers and optical fiber cables are becoming, or have become, the transmission media of choice, primarily because of the tremendous bandwidth capabilities and low loss of such fibers.
Unfortunately, optical fibers, themselves, are quite delicate and can be easily broken or cracked to the extent that the signal transmission characteristics of the fiber are impaired. Further, when numerous fibers are contained in a cable, they are subject to stresses and strains when the cable is laid, especially when it is pulled or sharply bent. To prevent such potential damages, cables have been designed to allow pulling and bending thereof without unduly stressing the individual fibers contained within the cable. However, even in such cables, the fibers can be damaged by storms, rodent burrowing, shifting earth, or by accidents during excavation and laying, as well as by subsequent digging, as with a backhoe, in the area of the laid cable. Aerial mounted or strung cables are likewise often subject to severe stresses, primarily from storms and the like.
In telecommunication systems, the amount of optical signal traffic typically carried by just one pair of fibers in a cable can generate thousands of dollars per minute for the operator. It follows, therefore, that there is a compelling need for monitoring an optical fiber telecommunication system in order to ascertain the occurrence of an event, or predict the occurrence of an event which impairs signal transmission on a fiber and, equally important, to ascertain the location within the system of the event. Inasmuch as every minute of down time for a fiber can result in considerable monetary loss, the elapsed time involved in precisely locating the event should be minimized as much as possible.
There have been numerous arrangements proposed for monitoring an optical fiber signal transmission system, with elapsed time from occurrence of the event to precise location of the fault ranging from five minutes to seventeen hours, for example. In general, the emergency response procedure is as follows. When an alarm sounds at a control center, the operators are made aware that there is a problem, i.e., event somewhere in the network. After a verification of the integrity and operation of the transmission equipment, a conclusion is reached that the problem is with the cable, and an emergency crew is dispatched in the general direction of the fault. The crew is usually, or should be, equipped with an optical time domain reflectometer (OTDR), a test instrument that generates its own optical signal, launches it into the fiber, and measures the elapsed time of reception for the signal reflection from the fault. The elapsed time affords a measurement of distance from the reflectometer to the fault. The crew connects the OTDR to the cable, hopefully in the vicinity of the fault or event, and obtains a reading of the optical distance to the fault. With the aid of a map of the network, the crew can then precisely determine the geographic location of the fault or event. A typical cable break can take hours to locate using the foregoing procedures, and as a consequence, emphasis has been on reducing this elapsed time to a minimum.
One proposed system, the Fiber Check 5000 of Photon Kinetics, as shown in a 1001 marketing brochure, is a sophisticated monitoring and test system utilizing three basic components, a control center, a plurality of "acquisition units" and a plurality of optomechanical switches, all of which are fixed in place throughout the system. The system controller is a CPU or computer having the capability of an optical time domain reflectometer and of maintaining a data base describing the route of each cable and fiber identification. Each acquisition unit, generally located in a central office, exchange, or the like, is functionally equivalent to an OTDR, with its test results being communicated to the CPU via modem. When an event occurs, the CPU can page the various acquisition units until the fault is located. The OTDR function enables the acquisition unit in combination with the CPU to pinpoint the precise location of the event, and the electro-mechanical switches, located with the acquisition unit, make it possible to test each optical fiber in the cable at the event site, or at the acquisition site closest thereto, to ascertain which one or ones are faulty. Such a system is capable of rapid location of a fault or other event, however, it relies heavily on a large number of fixed components essential to its fault detection, fault location and monitoring functions.
Present day monitoring and fault location systems necessarily include some means for testing individual fibers in order to ascertain exactly which fiber or fibers have a fault, such as a break, and such testing is generally made possible through the use of light guide cross-over switches. An example of such a system for testing individual fibers is disclosed in U.S. Pat. No. 5,329,392 of Cohen, wherein the apparatus for switching an OTDR among several individual fibers is shown. In the Cohen system, a monitoring component is placed between the external optical fibers portion of a fiber optic terminal system and the internal fibers within a central office. The monitoring component consists of planar "main" waveguides formed on a substrate which connect to the individual fibers. Monitor waveguides are connected, by means of directional couplers, to individual monitoring devices. In addition, OTDR signals are applied to the main waveguides by means of wavelength division multiplexors (WDM), which are connected through a 1.times.N optical switch to the OTDR, and the output of the waveguide component is connected through a M.times.N cross connect switch to the external fibers leaving the central office.
There are numerous arrangements in the prior art for switching among optical fibers, to achieve M.times.N switching, where M and N=1,2,3, - - - . One preferred switching system is the so-called moving fiber switch, which utilizes external forces to change the location of the fibers within the switch. A switch of this type is shown and described in U.S. Pat. No. 4,946,236 of Dantartas, et al., which functions as 1.times.2, 2.times.1, 2.times.2 switch. In that switch, the fibers are physically moved by means of magnetic forces to one of two positions, thereby achieving cross connection. A more versatile switch structure having a 1.times.N capability is shown in U.S. Pat. No. 4,986,935 of Lee. In that switch, an array of N fixed fibers is arranged in a semi-circle around a rotatable member having a single fiber mounted therein and a pivot axis aligned along the central axis of the semi-circle. The rotatable member is rotated by a stepping motor and the angular orientation of the fixed fibers is such that the single fiber is optically aligned with any selected one of the fixed fibers by means of the stepping motor. In order to assure adequate signal coupling, appropriate lenses on the ends of the fibers are used to expand or to collimate the light. As a consequence, the single fiber is switched to the desired fixed fiber upon proper command to the stepping motor. Such a 1.times.N switch is suitable for use, for example, with the arrangement of the Cohen patent for switching the OTDR signal to the desired waveguide for testing.
From the foregoing, it can be appreciated that the switch arrangements of the prior art as used in fiber monitoring and testing, are limited simply to switching among the various fibers in order to connect the monitoring apparatus thereto. Further, it is still necessary in prior art systems that the monitoring be performed by fixed elements strategically located throughout the system. OTDR based remote fiber testing systems (RFTS) generally require approximately three minutes for trace acquisition and processing per fiber. Thus, a worst case scenario for a twenty-four fiber cable test is seventy-two minutes, which, as pointed out heretofore, can result in considerable lost revenue. Most RFTS systems that are linked with transmission system performance hardware must be adapted to the protocols of each manufacturer's proprietary system, which is especially undesirable from an economic standpoint. Independent and distributed surveillance architecture is much to be preferred. Typical commercially available RFTS are based on centralized dam acquisition and processing platforms, whereas a distributed surveillance architecture allows greater flexibility and is more fault tolerant.